1. Field of the Invention
The present invention relates to a surface evaluation method and an associated surface evaluation system, and more particularly to light source technology for non-contact evaluation of a surface of a silicon melt (molten silicon) using optical phase-shift interferometry.
2. Background of the Invention
Phase-shift interferometry is proposed as a technology used to perform a non-contact evaluation of the condition of the surface of a sample. Phase-shift interferometry is described, for example, in Applied Physics Vol. 63, No. 7, page 713 (1994). In this technology, interference is caused between an evaluation signal reflected on the surface of the sample and a reference signal, and the sample surface is analyzed. Light generated from a He-Ne (helium-neon) laser light source is divided into an evaluation signal and a reference signal using a splitter, the evaluation signal being shone onto the sample surface and the reference signal being shone onto a reference surface, after which light is collected and interference caused. In the prior art, this technology was applied to the evaluation of samples such as crystal growth surfaces and molten surfaces, and the reference surface was generally a plane mirror.
The above-noted prior art, however, in the case in which the sample is non-planar, such as with a molten cylinder of silicon, did not provide sufficient technology or systems to perform evaluation of the surface of the molten fluid. That is, when using a reference surface with conditions that are different from those of the sample surface, unnecessary information are included in the interference signal, thereby making it difficult to evaluate the sample surface of interest.
In phase-shift interferometry in the prior art, unnecessary radiated wave and unnecessary reflected wave emanate from the surface of the sample, and when the frequency of the above-mentioned unnecessary radiated wave and the reflected wave are almost same as that of the evaluation signal, the evaluation signal is influenced by the unnecessary radiated wave and reflected wave, making it impossible to obtain a precise evaluation. In the case in which the sample is a silicon melt which is heated by a lamp using an image furnace in particular, the spectrum of radiated waves from the silicon melt and the spectrum of reflected waves of the lamp filament image from the silicon surface are approximately same with that of black-body radiation. For this reason, with a He-Ne laser of the prior art (633 nm), the influence of the above-mentioned radiation and reflection was too large to ignore.
Accordingly, it is an object of the present invention to provide a surface evaluation method and associated surface evaluation system that enable highly precise surface evaluation, without influence from a radiation signal from the sample.
According to the first embodiment of the present invention, a surface evaluation method in which evaluation signal from a light source is split to obtain a divided evaluation signal and a reference signal, the divided evaluation signal and the reference signal being shone, respectively, onto a sample surface and a reference surface, after which a reflected divided evaluation signal and a reflected reference signal, which are reflected respectively on the sample surface and the reference surface, are caused to interfere for analysis of the sample surface, and the reference surface is made of the same material as the sample.
According to the first embodiment of the present invention, the reference surface has the same curvature as the sample.
According to the first embodiment of the present invention, the sample surface has a shape of a side surface of a cylinder, and the reference surface a shape of a side surface of a cylinder.
According to the first embodiment of the present invention, the reference signal is reflected at a reference surface that is a polished mirror surface.
According to the second embodiment of the present invention, in a surface evaluation method for evaluating a sample surface having curvature, wherein an evaluation signal from a light source is divided so as to obtain a divided evaluation signal, the divided evaluation signal being shone onto the sample surface at a direction that is normal thereto.
The second embodiment of the present invention further provides a surface evaluation method, wherein an incidence onto a reference surface of a reference signal obtained by dividing an evaluation signal from a light source is in a normal direction with respect to the reference surface.
According to the third embodiment of the present invention, in a surface evaluation method in which evaluation light from a light source is split to obtain a divided evaluation signal and a reference signal, the divided evaluation signal and the reference signal being shone, respectively, onto a sample surface and a reference surface, after which a reflected divided evaluation signal and a reflected reference signal, which are reflected on the sample surface and the reference surface, respectively, are caused to interfere for analysis of the sample surface, wherein a frequency of the evaluation signal is selected such that a frequency of the reflected divided evaluation signal is different from that of a radiation signal emanated from the sample surface.
The third embodiment of the present invention also provides a surface evaluation method in which, after obtaining an interference signal by causing interference between the reflected divided evaluation signal and the reflected reference signal, a signal component having a wavelength that is longer than that of radiation signal is removed from the interference signal.
The third embodiment of the present invention also provides a surface evaluation method in which the wavelength of the evaluation signal is shorter than that of the radiation signal.